Various methods and devices have been proposed for ultrasonically scanning a volume within a subject for three-dimensional imaging and display. Many of these techniques involve the scanning of a number of spatially adjacent image planes of a region of the body. The ultrasonic information from these associated planes can be analyzed and displayed on the basis of spatial coordinates of the data within a plane, and on the basis of the spatial relationship of each plane to the others. The information can be displayed in a three-dimensional image format such as a perspective view of the volume being imaged. A number of scanning techniques utilizing specially devised scanning devices have been proposed for acquiring these spatially related image planes. It is preferable, however, to be able to acquire information data for three-dimensional presentation without the need for special scanning devices or apparatus.
It has therefore been proposed to use ultrasound phased array imaging systems comprising a transducer array probe with transducer elements disposed in a 2-D plane for forming 3-D images of a region of a body. These known systems can provide real time 3-D gray images by acquiring data in a pyramidal volume whose boundaries lean onto the 2-D phased array perimeter and form angles of about 30°×30°. These 3-D images permit of examining in real time a body structure comprised in this pyramidal volume. In fact, these 3-D gray images are realized in real time with a frame rate of about 20 Hz. These 3-D gray images correspond to the reference volume scanned by the 2-D phased array probe where beam formation steps are performed at the level of the probe, allowing the synthesis of four simultaneous receive beams. Doing motion extraction in three dimensions and in real time presents additional problems to solve.
Now, conventional 2-D imaging systems for providing real time Doppler images of moving parts of a body are already known. Using said 2-D imaging systems, Doppler imaging requires analyzing typically eight successive transmit-receive signals in order to extract and process flow signals. With the 3-D systems previously described, real time 3-D flow imaging would necessitate an acquisition time that would be eight times longer than the time necessary to acquire 3-D gray images of a structure, achieving a frame rate of about 2 Hz. Another solution for acquiring real time 3-D Doppler flow images would be to acquire a volume eight times smaller than the pyramidal reference volume during one cardiac cycle. In this case, eight successive triggered cardiac cycles would be necessary to reconstruct the reference pyramidal volume. Hence, the known systems do not permit to form 3-D Doppler images of fluid flow or tissue motion of a body in real time because this operation would necessitate multiplying the acquisition time by eight in such a reference volume or would necessitate scanning a volume eight times smaller. In each case, the real time quality of such 3-D images would be unsatisfactory.
Because flow imaging allows to image coronary trees or vessels of organs such as liver and kidneys, etc., it is crucial to provide means that allow extending ultrasound modality to 3-D Doppler flow imaging in real time.
Ultrasonic images are subject to image artifacts arising from a number of sources such as reverberation, multipath echoes, and coherent wave interference. These artifacts will manifest themselves in various ways in the images, which can be broadly described as image tissue. The image tissue becomes particularly troublesome when images are presented in a three-dimensional format, as the three-dimensional tissue can interfere with a region of the body and obscure said region, which the clinician is attempting to visualize. Moreover, strong anatomic structures like arterial walls or cardiac walls mask the weak signals generated by blood. When imaging blood flow, these structures represent the tissue. Accordingly, it would be desirable to provide ultrasonic image information in a format in which tissue does not significantly impair the images of the body region. For example, it would be desirable to provide ultrasonic image information in a format in which tissue information may be separated from flow information.
It is already known to image the body using Doppler information. Doppler information has been used to image the body in two distinct ways. One Doppler imaging technique is commonly referred to as Doppler velocity imaging. As is well known, this technique involves the acquisition of Doppler data at different locations called sample volumes over the image plane of an ultrasonic image. The Doppler data is acquired over time and used to estimate the Doppler phase shift or frequency at each discrete sample volume. The Doppler phase shift or frequency corresponds to the velocity of tissue motion or fluid flow within the body, with the polarity of the shift indicating direction of motion or flow. This information may be color coded in accordance with the magnitude of the shift or velocity and its polarity, and usually overlaid over a structural image of the tissue in the image plane to define the structure of the moving organs or flowing fluids. The colors in the image can provide an indication of the speed of blood flow and its direction in the heart and blood vessels, for instance.
A second Doppler technique is known as power Doppler. This technique is unconcerned with estimations of the velocity of motion or fluid flow. Rather, it focuses simply on the intensity of the received signals that exhibit a Doppler shift. This Doppler signal intensity can be measured at each sample volume and displayed in a color variation. Unlike Doppler velocity imaging, power Doppler does not present the problems of directionality determination and low sensitivity that are characteristic of velocity imaging. Color power Doppler simply displays the Doppler signal intensity at a sample volume in a coded color. Like color Doppler velocity imaging, the color power Doppler display is conventionally displayed with a structural B mode image to define the organ or tissue structure in which motion is occurring. Since the value at each sample volume can be averaged over time or based upon a peak value, and is not subject to the constant changes of velocity and direction which are characteristic of Doppler velocity signals, the color power Doppler display can be presented as a stable display of motion or flow conditions in the body.